This invention relates generally to the detection of a presence of a characteristic in an object, and, more particularly to the application of sequenced technologies to detection of a presence of a characteristic in an object.
Detection of the presence of a characteristic in an object is a common function in a variety of applications. Exemplary applications, while not being an exhaustive listing, include detection of a particular disease at a location in a patient, detection and tracking of targets in military applications, detection of obstacles and objects in robotics and in automotive applications such as self closing sunroofs or doors.
An example of detection of the presence of a characteristic in an object is the detection of an undesired substance within an object, such as detection of explosives inside objects. Recently there has been increased awareness of the potential for large-scale introduction of explosives to create chaos or to harm an intended set of victims. There are currently a variety of technologies available to detect the presence of explosives. However, systems utilizing a single technology do not effectively measure all explosive types or meet the desired accuracy. In general, systems utilizing a single technology do not usually meet the desired conditions for the detection of a presence of a characteristic in an object. Combining technologies will not always result in improved performance. Data Fusion techniques can be used to provide guidance in the combination but such techniques require large computational resources and/or learning strategies.
There is a need for simpler strategies for interconnecting detection systems, where each system utilizes a single technology, hereinafter referred to as sub-systems, and for combination systems designed according to that simpler strategy.
Background on Explosive Detection Technologies
Since an important example of detection of the presence of a characteristic in an object is the detection of explosives inside objects, in order to better understand the descriptions of systems and methods for the detection of explosives inside objects, a background discussion of explosive detection technologies is provided herein below.
Explosive Detection Technologies
The Explosive Detection Systems (EDS) considered are based on X-ray technologies, Neutron-Based Bulk Explosives Detection technologies, Other Nuclear (non-neutron) Bulk Explosive technologies, Nuclear Quadrupole Resonance technologies, and Trace Detection technologies.
X-Ray technologies
X-ray Imaging, Dual Energy X-ray Imaging, Computed Tomography (CT), Dual Energy CT, backscatter radiography, and diffraction radiography all rely on x-rays penetrating into objects being transmitted, absorbed, scattered, or diffracted. Each of these processes has been exploited as an explosive detection system (EDS) technology.
X-Ray Imaging
Projection x-ray imaging uses a conventional x-ray source exposing objects in tubs or sacks. The portion of the x-rays that pass through the objects are detected and converted to digital data and a visible image. The attenuation of x-rays is described by(1−Io/i=e(μx) where Io is the output x-ray intensity, Ii is the input x-ray intensity, e is the natural log base, μ is the linear x-ray attenuation coefficient (1/cm), and x is the x-ray path length (cm). The μx is actually the sum of the linear x-ray attenuation coefficients of each material in the x-ray path multiplied by the path length of each material. Each linear x-ray attenuation coefficient is a function of the material density and effective atomic number (Z). Many explosives have a density about 1.6 g/cm3 and similar materials (N, O, C, and H) so explosives tend to look similar in x-ray images. Unfortunately, many other materials have linear x-ray attenuation coefficients similar to explosives, including organics and plastics. A thin high density material can look the same as a thick low density material. Although projection x-ray imaging is not used for explosive detection, x-ray imaging can be used for identifying shapes and objects related to bombs such as detonators, timers, batteries, and wires. Although such objects can be hidden using an x-ray absorbing material, such as lead, to block the x-rays, the lead shield would be visible in x-ray images and would cause a lack of penetration alarm.Dual Energy X-Rays Imaging
The linear x-ray attenuation coefficient of each material is a function of the x-ray energy, density, and effective atomic number. If x-ray image data is acquired with two or more x-ray energies (KV's) then for each picture element (pixel) the density and effective atomic number can be calculated. Using the density and effective Z gives improved ability to discern explosives from similar materials. The density and effective Z are still averaged over the entire path of the x-rays, so a thin high density material can look the same as a thick low density material.
Computed Tomography (CT)
CT data is acquired by measuring the x-ray attenuation thru the object being examined from many different positions and angles. The x-rays can form a cone beam, fan beam, or pencil beam. The x-rays can be detected as an area, a strip, a line, or a single point. An x-ray source emits x-rays that are attenuated as they pass through the object being examined. The intensity of the x-rays exiting the object are measured by the detectors, digitized, and fed to a computer along with position information. The computer uses the x-ray three dimensional vector information to determine the location of each voxel (volume element) that each x-ray passed through. The process called back projection adds part of each x-ray measurement to each of the voxels (volume elements) that the x-rays passed through. The result is a 3 dimensional (3D) reconstruction of the object being examined. The linear x-ray attenuation coefficient, μ, is estimated for every voxel. More x-rays and more angles passing thru a voxel give better estimates of u. CT provides a 3D array of measured x-ray attenuation coefficients as a function of position. As with projection x-ray, the x-ray attenuation coefficient is directly related to the material in the voxels. CT has an advantage over projection x-ray in that instead of averaging over the entire x-ray path, there is an x-ray attenuation coefficient for each voxel. Smaller voxels make it less likely that more than one material is in the voxel and likelihood of correctly interpreting the material is improved. The CT x-ray source and detector can be mounted on a gantry, which rotates around the object being examined. Similarly, the CT x-ray source and detector can be fixed where the object being examined is rotated. Typically, a fan beam is used with a line detector on a gantry to gather slice data and the object is translated on a conveyor to acquire multiple slices. It should be noted that the CT X-ray source could also be a cone beam source. Furthermore, it should be realized that the above embodiments are sample embodiments and that other embodiments are possible. If the conveyor is fixed during acquisition the result is a slice and if the conveyor is moving the result is a helical scan.
Alternatively, the CT can use multiple x-ray sources and multiple detector arrays to eliminate the need for a rotation. Fix source CT has advantages of less moving parts, higher speed, higher mechanical reliability and disadvantages of lower electrical reliability, more electrical parts, less accurate reconstructions due to fewer measurements. Each configuration requires specialized algorithms for back projection and reconstruction. All CT configurations measure the x-ray attenuation coefficient for each voxel, however some systems produce small voxels covering the entire volume while some have space in between the voxels and some systems only look at the parts of the volumes that were found to be suspect by other technologies. All are types of CT, but their capabilities vary widely.
Dual Energy CT
Two sets of CT data acquired using two different x-ray energies can be used to generate reconstructions for the two energy levels. The linear x-ray attenuation coefficients for the two energies for each voxel can be used to solve for the density and effective atomic number of each voxel. The density and effective atomic number can give a more accurate indication of the material in the voxel than just x-ray attenuation coefficients of single energy CT. Dual Energy CT (DECT) is more effective than dual energy projection x-ray because multiple materials are less likely to be found in a voxel than in an entire x-ray path of projection x-ray images. Multiple energies can be used to further enhance the data quality, but not by much. X-ray sources generally emit wide spectrums of x-ray energy that tend to overlap and blur the value of more than two energies. The compromise with DECT is that widely separated energies work better, but widely separated means that one energy must be lower energy and the lower energy may not penetrate the object being examined as well. The number of x-ray photons may be too low to give a useful improvement in the probability of detection and probability of false alarm for explosive detection in some objects.
Backscatter Radiography
When x-rays enter a material some of the x-rays are absorbed by materials and reemitted (scattered) at lower energy in all directions. Those x-rays returning to near the x-ray source are called backscatter. A collimated pencil beam x-ray source is used to determine the location doing the backscattering. Scanning the x-ray pencil beam can produce an image of the backscattered x-rays. Backscatter images are particularly good at imaging lower density materials such as organics, plastics, and explosives. Backscatter imaging can not distinguish between the material types. Scanning pencil beam imaging systems, such as those used in backscattering imaging systems, may simultaneously produce a transmission or projection x-ray image which is useful in for identifying shapes and objects related to bombs such as detonators, timers, batteries, and wires. Backscattering is a self-filtering process so the farther into the object being examined the x-rays travel, the less sensitive backscatter image is.
Diffraction Radiography
When x-rays entering a material encounter crystalline materials, the x-rays are bent or diffracted in a direction dependent on the material. Some explosives components are crystals and can be detected by the diffraction angle of the x-rays. X-rays are diffracted by many other common materials including metals, sand, gems, medicine, some papers, and photos. Thick materials give wide diffuse diffraction angles making it hard to distinguish one material from another. Simply detecting crystalline materials gives insufficient explosive detection specificity. This is another technology that is useful in the lab and with special test objects, but may be difficult to use in a real application.
Neutron-Based Bulk Explosives Detection Technologies
The basic principles behind the neutron-based Explosives Detection Systems (EDS) are the following: (1) a beam of neutrons enters the package to be inspected, (2) the neutrons interact with nuclei present in the explosives, (3) as a result of the interaction, either (a) new radiation, characteristic of the target nucleus, is produced, and/or (b) a deflection or absorption of the neutron beam occurs, (4) the new radiation and/or the neutron beam exiting the target is detected, and (5) computer analysis of the outgoing particles is used to deduce the presence of the explosive. All of the neutron-based techniques try to detect explosives by detecting and quantifying the atomic components of explosives, nitrogen (N), oxygen (O), carbon (C) and hydrogen (H). The main signature for explosives is the relatively high concentration of nitrogen and secondarily oxygen, and the relatively low concentration of carbon and hydrogen, as compared to common materials. The various neutron-based technologies differ in the radiation (referred to in step 3, above) they use to deduce the presence of one or more of N, O, C and H, the properties of the neutron beam whose interaction produces the radiation, and the method, if any, used to localize (i.e., image) those materials within the package being inspected. In the following, a brief description of the theoretical basis of each of the neutron-based technologies is provided, starting with the most mature technology and ending with the least.
Thermal Neutron Analysis (TNA)
In TNA the detected radiation is a gamma ray. The neutron interaction producing that gamma ray is the capture of the neutron by the nucleus; for example, the isotope 14N captures a neutron to become 15N. Although 15N is a stable isotope, it is initially produced in a very highly excited state, which subsequently (within 10−17 s of the capture) decays to its stable state, producing a number of gamma rays, the most energetic of which has energy of 10.8 MeV. The probability of neutron capture is (usually) highest for the slowest neutrons; the slowest that a neutron inside a target can be is when its energy is the same as the average thermal energy of the target, hence the name TNA. The cross section for thermal capture on 14N is 75 mb1, on 16O 0.19 mb, on 12C 3.53 mb, and on 1H 333 mb. (The cross section is a measure of the likelihood that a process take place; the above numbers indicate that for an equal number of atoms, a thermal neutron is 75 mb/0.19 mb=394 times more likely to be captured by a 14N nucleus than by an 16O nucleus, and 333 mb/75 mb=4.4 times as likely to be captured by a 1H nucleus than by a 14N nucleus.) These cross sections indicate that TNA is primarily sensitive to the N component of explosives and not to the O content. Thus the presence of relatively small quantities of N-rich substances such as Polyurethane or Melamine, or large quantities of substances containing small-to-moderate concentrations of N, such as Orlon, silk and wool, can trigger a false alarm in a TNA system. 1 For N, thermal capture produces a 10.8 MeV gamma ray only 14% of the time, so the effective cross section for that signature is 0.14*75=11 mb.
The neutron source for TNA could be a radioactive source or an accelerator based source (the term Electronic Neutron Generator, or ENG, is sometimes used in the industry to refer to accelerator-based neutron sources). The most suitable radioactive source for TNA is the fission source Californium 252, 252Cf, which produces 2.30×106 n/s per microgram. The energy distribution of the emitted neutrons peaks between 0.5 and 1.0 MeV and extends to as high as 10 MeV, with the intensity dropping almost exponentially with increasing neutron energy. Thus the size of the source is determined by the encapsulation requirements and the amount of moderator needed to bring down the energy to thermal levels. The half-life of 252Cf is 2.6 years.
Accelerator based neutron sources for TNA make use of either the D—D (deuteron-deuteron) reaction: 2H+2H→3He+n, or the D-T (deuterium-tritium) reaction: 2H+3H→4He+n. The deuteron (nucleus of the deuterium heavy isotope of hydrogen) is accelerated by a potential of 100–300 kV and the accelerated beam usually impacts a deuterated or tritiated target, such as titanium deuteride, or titanium triteride. The D—D reaction releases 3.26 MeV of energy and the D-T reaction releases 17.6 MeV of energy. Because the released energy is much larger than the beam energy, all outgoing neutrons leave with about the same energy, approximately 2.7 MeV for the D—D reaction, 14 MeV for the D-T reaction. A 1 mA beam of deuterons will produce about 109 n/s for D—D reaction and 1011 n/s for the D-T reaction; somewhat smaller yields are produced from compact sealed-tube neutron generators. Because of the depletion of the target, sealed tube generators have operating life times of less than 2000 hours. As in the case of a radioactive source, the neutrons are produced isotropically and hence one can make use of only those neutrons that end up in the direction of the target to be inspected.
The gamma rays produced in TNA are usually detected with NaI(Tl) or BGO crystals. The gamma-ray spectrum usually consists of a full-energy photo peak plus two escape peaks plus a continuum Compton background. For the energies encountered in TNA and FNA (see below) typical sizes are 4″×4″×4″ per crystal; while larger size crystals do exist, larger sizes result in higher count rates and in TNA could easily reach a MHz, which is probably the upper limit for single pulse counting with NaI. One then employs as many crystals around the package being inspected as geometry and cost allow.
Other scintillating crystals, such as BGO, have a higher efficiency than NaI, but have a lower energy resolution and a higher cost. Given the potential complexity of the gamma-ray spectra, the better energy resolution of NaI crystals favors their use.
Fast Neutron Analysis (FNA)
As in TNA, the detected radiation signaling the presence of an explosive is a gamma ray characteristic of one or more of its atomic components. The neutron interaction producing the gamma ray is, however, not thermal capture, but inelastic neutron scattering. In this process an energetic (i.e., fast, hence the name FNA) neutron collides with a nucleus and transfers some of its energy to that nucleus. The nucleus then decays back to lower excited states giving off one or more characteristic gamma rays. The minimum neutron energy needed to excite the first gamma-decaying state of 12C, 14N, and 16O is 4.4 MeV, 2.3 MeV, and 6.2 MeV, respectively. One of the advantages of FNA over TNA is its sensitivity to 16O and 12C, in addition to its sensitivity to 14N. This sensitivity comes about because the cross section for inelastic scattering from those atoms is much larger than the cross section for thermal neutron capture. For 14.8 MeV neutrons (i.e., those produced by the D-T reaction) the cross section to excite the above states is 200 mb for 12C, 70 mb for 14N, and 100 mb for 16O. However, since the minimum neutron energy needed to excite the oxygen is 6.2 MeV, one cannot use the radioactive source 252Cf as the neutron source; one must use either the D-T reaction, which produces 14.8 MeV neutrons for an accelerator that provides the minimum of −200 keV of energy to the accelerated deuteron, or the D—D reaction, with an accelerator that provides at least 5 MeV to the deuteron. (Other means of producing fast neutrons exist, but only the methods employed in machines produced purely for explosives detection using neutron-based technology are described herein.)
The gamma-ray detectors used in FNA are of the same type used in TNA, (mainly NaI) since the energies of interest are of the same order, with FNA perhaps affording the use of slightly smaller detectors since the maximum energy of interest (6.13 MeV from 16O) is smaller than that encountered in TNA (10.8 MeV from capture on 16N).
The improvement in FNA over TNA comes about because of the enhanced sensitivity to 16O, whose density is highly correlated with that of N in explosives (both O and N content is high in explosives). However, it is possible to get a false identification if it happens that there are two (or more) objects in the package being inspected one with high N content and the other with high O content, with the ratio being close to that of an explosive. To reduce the probability of such false positives, it is desirable to add imaging capability to the above techniques, i.e., is it is desirable to add the ability to localize the position of the N and O signatures within the package. The theoretical basis for variations of the FNA and TNA techniques, designed to add imaging capability and/or improve their sensitivity, is presented herein below.
Short Pulse FNA (SPFNA, also Known as Nanosecond Pulse FNA or Simply as PFNA).
This technique is based on FNA. The deuteron beam producing the neutrons through its interaction with a deuterium target is pulsed, i.e., the beam particles are bunched together (by electromagnetic means) into pulses each about 1 ns (10−9 s) in duration and the pulses repeated about 1–5 MHZ, i.e., the time between one pulse and the next is about 200–1000 ns. The reason for this short pulsing is to determine where the neutron interacted in the package being inspected, and hence to localize the source of the gamma rays (and hence the source of the O and N). The localization along the direction of travel of the neutron is deduced as follows: the electronics that produce the deuteron pulse in the beam generate a fast time signal with each pulse. This time signal starts an electronic timer; when a gamma ray is detected in the NaI (say) the generated pulse is used to read the timer. (In practice the timer is started on the NaI signal and stopped on a delayed version of the beam pulse signal.) The measured time is linearly proportional to the distance traveled by the neutron and hence gives the position along the neutron travel direction where the scatterer is located. (The effect of the additional time taken by the gamma ray to travel from the neutron interaction site to the detector is discussed below.) The position in the two directions perpendicular to the neutron travel direction is determined by sufficient collimation of the neutron beam and by rasterizing the beam along one of the two directions (say the vertical direction) while a conveyor belt carrying the package being inspected and moving at a known speed rasterizes the object along the horizontal direction.
The position resolution (along the beam direction) is determined by the speed of the neutron, the duration of the pulse, and the time resolution of the NaI-electronics combination. For an 8.4-MeV neutron, the speed is 2.8 cm/ns; for a NaI detector the time resolution is at best 1 ns; when combined in a statistical way with the 1 ns pulse width the resulting time resolution is probably at best 1.4 ns, leading to a position resolution of about 2.8 cm/ns*1.4 ns=4.0 cm. (=1.6″). For a 14.8 MeV neutron, such as would be produced by a low-voltage D-T accelerator, the neutron speed is 3.7 cm/ns and the corresponding resolution would be 2.1″. The resolution in the direction perpendicular to the beam direction is determined primarily by the collimation of the neutron beam and other geometrical considerations. Because of the much larger speed of gamma-rays (the speed of light=30 cm/ns), the additional time taken by the gamma ray to travel from the point of interaction to the detector is small compared to that taken by the neutron to travel within the package; nevertheless, with a knowledge of the position of the detector, its contribution can be taken into account with sufficient accuracy so as not to degrade the position resolution.
One factor contributing to the degradation in position resolution is scattering of the neutron prior to its gamma-registering interaction; the probability of that interaction depends on the size and content of the package being inspected.
Long Pulse FNA (LPFNA, Microsecond Pulse FNA or Gated FNA, or Gated FNA/TNA)
Like in SPFNA, the deuteron beam producing the neutrons is pulsed, however the pulse duration here is on the order of several microseconds (typically 10–50 μs), and the separation between pulses is on the order of 100–1000 μs. The reason for the pulsing here is not to obtain position information (the pulse duration and the resulting spread in interaction positions is far too large to provide useful imaging), but rather to improve the sensitivity and accuracy of the gamma-ray measurement. This is done by separating those gamma counts that arrive during the pulse (i.e., with the beam on), from those that arrive in between pulses (beam off). The former are then largely due to fast neutron interactions (i.e., close to what would be observed in an idealized FNA measurement), and the latter largely due to thermal neutron interactions (i.e., close to what would be observed in an idealized TNA measurement). In principle one can also employ a “pure” gated TNA technique (GTNA), in which the neutrons are thermalized with a moderator before they get to the package and then only count gammas during the beam off period. It should be clear that in either case, the pulsing is still too short to be done by mechanical means and hence only electronically generated neutrons (i.e., accelerator-based) can be used.
Neutron Resonance Attenuation (NRA)
In this technique a ns-pulsed beam of neutrons, with a broad energy spectrum (0.5–5 MeV) enters the package to be inspected from one side, and the neutrons that pass through the package are detected on the other side. The elements making up explosives (N, O, C, H) have large scattering cross sections at specific energies (that is resonances in the cross section) and hence, if present, would scatter away neutrons of that energy from the beam (i.e, the beam component at that energy would be attenuated). Therefore by looking for depletions in the transmitted neutron beam at characteristic energies the presence of explosives constituents could be deduced; the depth of the depletion provides a quantitative measure of the total areal density (density×thickness) of the scattering element along the beam direction. By rasterizing the beam (and/or the target), a two-dimensional shadowgram of the elements of interest can be formed. The neutron energy is usually measured with a Time-of-Flight (ToF) technique: the beam pulse starts a timer and the neutron detector pulse stops the timer. The measured ToF then gives information on the velocity of the neutron (and hence its energy), since the distance between the neutron source and the detector is fixed. The sensitivity of the technique depends, among other factors, on the energy resolution, which, in turn, depends on the length of the flight path, with longer flight paths giving higher resolution. Typical flight paths are 2–10 m long.
Associated Particle Imaging (API)
This technique uses the neutrons to interrogate the target in essentially the same way FNA does: a fast neutron excites the N, O and C nuclei and a subsequent gamma characteristic of these nuclei is detected. The imaging is done in a different way than in SPFNA, though. The reaction used to produce the neutron is the D-T reaction: 2H+3H→4He+n. In this reaction the direction of the alpha particle (the nucleus of 4H) is correlated with the direction of the neutron (for the low beam energies typically used in the D-T reaction the alpha particle and the neutron are emitted essentially back-to-back). Therefore if the alpha particle is detected at a known direction (say by using a position sensitive detector and/or an array of detectors each with a small angular size) then the direction of the associated neutron is known. The position of the N, C, O along the (now known) direction of the neutron is obtained from the time difference between the alpha particle and the gamma ray.
While API would allow the use of a simpler accelerator than used in SPFNA, and would also allow a larger area of the target to be inspected at a time, the high count rates in the alpha detectors necessitate the use of lower beam currents and hence longer inspection times. While there may be ways to ameliorate the alpha count rate problem, for example by using a large number of small detectors each with a lower count rate, such systems have not, to our knowledge, been developed commercially yet.
Other Nuclear (Non-Neutron)-Based Bulk Explosives Detection Technologies
An overview of other nuclear-based techniques for explosives detection that do not rely on the generation and interaction of neutrons follows. The theoretical basis for two technologies that are under active commercial development, Gamma Resonance Technology (GRT) and Gamma Radiography, is presented herein below.
Gamma Resonance Technology (GRT)
As in Fast Neutron Analysis (FNA), GRT detects explosives by detecting the presence of one of the principal components of explosives, nitrogen. The nitrogen is detected by probing the package with a gamma ray whose energy is uniquely tuned to have an enhanced interaction probability with nitrogen. Further, the gamma-ray beam is produced in such a way that only nitrogen along a specified path interacts with the beam, thus allowing imaging of the nitrogen. The way these unique gamma rays are produced is as follows: A beam of protons with an energy of 1.75 MeV impinges on a 13C target. Some of the protons are captured by the 13C nuclei to form 14N in an excited state at 9.172 MeV. About 5% of the time that excited state decays by emitting a gamma ray with an energy of 9.17 MeV (the remaining 95% of the time the state decays back into a proton plus a 13C nucleus). Now the laws of physics, at the microscopic level, seem to be time reversible, so that if the process in which a stationary excited 14N nucleus decays to the ground state by emitting a 9.17 MeV ray happens, then its time inverse, namely the process in which a 14N nucleus in its ground state absorbs a 9.17 MeV gamma ray to become an excited 14N at 9.172 MeV excitation should also happen, with essentially equal probability. So the presence of 14N in the package being inspected can be inferred by monitoring the number of 9.17 MeV gamma rays passing through the package. A decrease in the number of transmitted gammas is indicative of the presence of 14N, potentially signaling the presence of an explosive. Now there is a slight complication in the physics which has both negative and positive consequences for explosives detection. The excited 14N produced by the beam is not stationary, but is moving in the direction of the beam (it has to carry the momentum brought in by the proton). Because of this motion, the gamma rays emitted in the forward direction (i.e., in the same direction as the proton beam) have more energy than can be absorbed by a nitrogen at “rest”, while gamma rays emitted in the backward direction have less energy and cannot excite the 14N to the excited state at 9.172 MeV. Consequently, there is a specific angle (relative to the proton beam direction) at which the gamma rays have just the right energy to be absorbed by the 14N. This angle is about 80.5°.2 The natural width of the 9.172 MeV level in 14N is 122 eV, so that gamma rays with about that much spread in energy can still be resonantly absorbed by the 14N; the spread in angle allowing that spread in energy is about 0.2°. 2 At that angle the energy of the gamma ray emitted by the moving 14N is 9.175 MeV, about 3 keV higher than the excitation energy of the resonant state in 14N. This extra 3 keV is needed because upon absorption of the gamma ray the 14N recoils back and hence takes up 3 keV of the energy of the gamma ray.
The negative consequence to the fact that only gamma rays emitted within less than half a degree from a cone with half angle of 80.5° from the beam direction is that only a small fraction of the emitted gamma rays (about 0.4%) fall within that range. The positive consequence is that the narrow resonant fan beam of gammas can be used to image the nitrogen content of a package as it passes through the slice, without the need for very precise collimation of the gamma beam.
There are two ways the presence of 14N in the package under inspection can be deduced: 1) a decrease in the number of counts of 9.172-MeV gammas along the direction of the cone. To be sensitive to that decrease, a detector capable of discriminating between the resonant gammas and the non-resonant gammas is needed. The principle used is essentially the same one used in detecting the 14N: a detector with 14N content is used; some fraction of the transmitted resonant gammas will be resonantly absorbed by the 14N in the detector; the 1.75-MeV protons from the decay of the excited 14N to p+13C are detected.
2) Detection of 9.1 MeV gammas resonantly scattered by the 14N away from the cone. Such a detector would be placed at relatively backward angles from the resonant gamma fan. The detector need only have moderate energy resolution for 9 MeV gammas, since the probability of non-resonant scattering is very small.
Gamma Radiography
Gamma Radiography employs exactly the same principle as x ray radiography: The variation in the absorption of a beam of photons as an object is scanned by the beam is used to from a shadowgram of the object. The shadowgram is produced because denser objects are more absorbing than less dense objects. In gamma radiography the gamma rays are produced by the decay of a radioactive isotope, instead of an x ray tube, or an electron accelerator. Typical sources used are 137Cs, which produces a 662-keV gamma ray, and 60Co, which produces two gamma rays, one at 1172 keV and the other 1332 keV. The higher energy of these gamma ray sources, compared to typical x ray sources, enables the examination of thicker targets; by the same token this higher penetration means that the contrast in the shadowgrams is relatively low.
Nuclear Quadrupole Resonance (NQR or QR)
Quadrupole resonance (QR, or more formally, nuclear quadrupole resonance, NQR) offers a highly specific method of detecting and identifying crystalline materials, including many explosives. Specially tuned pulses of radio waves in the 0.5–5 MHz range are applied to the sample. The radio waves are absorbed by nuclei of atoms in crystals within the sample and re-emitted a short time later. These weak re-emitted radio waves are detected by a sensor coil in the NQR receiver. The radio spectrum corresponds to particular atoms in a particular molecular structure, so each peak in the spectrum indicates a unique crystalline material.
Not all atomic nuclei respond to the QR radio pulses. The resonance effect is specific to nuclei with magnetic spin quantum numbers greater than or equal to 1, which includes Nitrogen-14 and other nuclei typically found in explosives. Another requirement for QR is that the nuclei must reside in an environment with a static or non-temporally averaged electric field gradient (EFG). What this means practically is that the nuclei must exist in an environment where the arrangement of surrounding charges remains fixed for at least a few microseconds. Nuclei in solid, amorphous materials could posses a QR spectrum, but that spectrum would tend to be broad and indistinct reflecting the presence of many different EFG environments. Nuclei in crystalline environments where atomic arrangements regularly repeat and the atoms don't move much will have narrower QR spectra as long the charge arrangement surrounding each nucleus gives a non-zero EFG. This describes the situation for many explosive and drug compounds. Nuclei in liquids do not possess a QR spectrum because the EFG is constantly changing and averages to zero much faster than 1 microsecond.
Although somewhat related to Magnetic Resonance Imaging used in medicine, QR is distinctly different in several ways. The most notable difference is that QR does not use a magnet to polarize the nuclei. QR polarization results from different energy states for the nuclei in the EFG. Not having to use a magnet is a major advantage of QR over competing techniques based on other magnetic resonance effects. Some of these advantages are lower cost, simpler design, and easier portability. The other difference between QR and MRI is that QR normally does not yield an image. A positive QR signal characteristic of an explosive indicates the presence of an explosive somewhere within the sample. Some degree of localization may be possible by taking advantage of the spatial distribution of RF field produced by specific antenna designs. Like MRI, QR uses RF radiation that is non-ionizing, so that it is generally safe for use on or near personnel.
The RF pulses are feed into a coil surrounding or adjacent to the sample, thereby producing RF fields within the sample. Likewise, the signal pickup coil must surround or be near the sample. In some cases, the transmit and receive coils are one in the same. Design of these coils as effective antennas for each NQR application and the associated electronics to handle and analyze the signals are major aspects of NQR technology development.
Because this technology uses radio frequency fields, RF shielding of the item under inspection becomes a possible means for defeat. Separate means can be devised to detect when an item being inspected contains a shielded region.
Trace Detection Technologies
(a) Trace Detection Technologies Summary
Intrusion into the package is required to limit false negative objects, i.e. not seeing a well packaged explosive, to an acceptable level. Once intrusion into the package is acceptable Trace Detection Technologies are appropriate to identify specific chemical explosive and appropriate emergency response. The various trace detection technologies are described in the following sections.
(b) Trace Detection Technology Description:
Trace Detection Technologies require physical transport of the sample (explosive) to the detector. Detectors only respond to (detect) gas phase chemical species. Particulate transported to a detector must go through a vaporization phase change prior to detection. Detection limits are best defined in absolute mass at the detector. Sampling, or transporting the chemical to the detector, is the most important limitation of trace technologies.
To illustrate the significance of packaging on detection explosive systems, two cases are described herein below.
The technology is adequately described by breaking the technology into three parts; sampling, sample preparation, and detection. The three independent parts are detailed in Table 1. First, define the analytical situation or problem, and then select one technique from each column appropriate to maximize and tailor the Trace Detection Technology to the problem.
TABLE 1Trace Detection Technologies have three independent parts that aremixed and matched to achieve analytical goalsSampling MethodSample PreparationDetectorsAir, human plumeNone or direct injectionIon Mobility SpectrometerAir, portalChromatographyMass SpectrometerAir w/absorbentCryo-fractionatingInfrared spectrometerpreconcentrationWipe sampleThermal desorptionElectron CaptureDetector (ECD)
Example detector sensitivities are listed in Table 2. These are estimates based on ideal laboratory tests. Detector sensitivity is often compound (chemical functionality) specific. In real world applications, significantly worse detection limits should be expected. Much of the sensitivity lost by taking instrumentation out of the laboratory is regained through a pre-concentration step as part of the sampling. Little difference in sensitivity is seen as a discriminator between manufacturers at these levels of detection once installed in the field.
Mass spectrometers (MS) and infrared (IR) spectrometers are generally considered separately from other detectors as they provided chemical structural information. This additional dimension to the detector provides significant advantages over nonspecific detectors. Using MS and IR spectrometers, the explosive in the object can be identified.
TABLE 2Detector Sensitivity LimitsLowerDetectionlimitLinearDetector Type(gm)RangeUseNonspecific Detector TechnologiesThermal10{circumflex over ( )}−6 10{circumflex over ( )}4GeneralConductivityFlame Ionization10{circumflex over ( )}−1010{circumflex over ( )}6OrganicElectron Capture10{circumflex over ( )}−1210{circumflex over ( )}2VariableChemiluminescence10{circumflex over ( )}−1010{circumflex over ( )}5Oxides of nitrogenSurface Acoustic10{circumflex over ( )}−1210{circumflex over ( )}5GeneralWaveCross-section20{circumflex over ( )}−6 10{circumflex over ( )}5GeneralIonizationPhosphorus10{circumflex over ( )}−1110{circumflex over ( )}4Organo-P CompoundsA little better than NonspecificIon Mobility10{circumflex over ( )}−1210{circumflex over ( )}4High ProtonSpectrometryAffinitySpecific Detector TechnologiesMass Spectrometer10{circumflex over ( )}−9 10{circumflex over ( )}4GeneralInfrared spectrometer10{circumflex over ( )}−9 10{circumflex over ( )}4General
Ion Mobility Spectrometer (IMS) detector are one step up from a nonspecific detector. What is compromised in resolution is gained in speed, usability, weight and cost. Within limits IMS can be used to identify chemical species through the use of computational analysis.
Chromatography cannot be used as a stand-alone detector technology. It can be used with most detectors and functionally prepares the sample, separates chemical species to facilitate the use of nonspecific detectors, or simplify computational analysis of MS and IR detectors when complex mixtures are sampled.
These technologies, and specifically MS/RGA with a thermal sample inlet, are appropriate to use when the package can be intruded upon and the specific chemical explosive needs to be identified to guide an emergency response.
Mass Spectrometry:
Mass spectrometer detectors fall into two types; bench units primarily used in laboratory settings and require highly skilled operators, and mobile units. Mobile units designed for gas analysis are often called RGAs (residual gas analyzers), and are often equipped with a thermal sample inlet. The sample inlet serves two (2) functions. First, sample is drawn into the sample inlet cold to concentrate chemical species and thus increase sensitivity. Second, the inlet is quickly heated vaporizing collected chemical species for introduction to the mass spectrometer detector allowing measurement of a variety of low vapor pressure chemical species at higher sensitivity.
The mass spectrometer detector has an electron ionization (about 70 eV) source. As chemical species enter the ionization beam positive ions form. The mass to charge ration of these ions and the uni-molecular decomposition, or fragmentation, products are measured. Every molecule has a unique mass fragmentation pattern that can be used to separate and identify it from any other compound. Greater computing power is needed when either more components are in the sample mixture or the more unknown chemical species are in the sample. To lessen the required computing power chemical species are separated or fractioned through the use of a short chromatographic column placed between the thermal inlet and the detector.
Mass spectrometer detectors greatest strength lies in its ability to provide structural proof of the chemical species. When screening level gets down to the point where chemical speciation is required (specific explosive or drug), this is the technology to use.
Ion Mobility Spectrometer (IMS)
IMS is well developed and in various forms has been around for years. Samplings offered by the listed IMS venders are air flow (human plumb or portals) or wipe, sample preparation is direct injection and detection is by IMS using atmospheric pressure chemical ionization (APCI).
IMS sensitivity is dependent upon the chemical species, generally proportional to higher proton affinity, and can range over several orders of magnitude (10−12 gm to 10−6 gm).
Ion mobility spectrometers use a radioactive source (about 15 milli-curies Ni63, some use Am241) to ionize gas molecules at atmospheric pressure (APCI atmospheric pressure chemical ionization). Ion clusters form and drift in a field, drift tube, being detected as they collide with spaced detector elements along the tube. The time of drift (mobility) is roughly a measure of the cluster mass. Often referred to as a time-of-flight measurement. Its limited chemical speciation is achieved based on the drift time or mobility in a positive and/or negative field. This is very oversimplified. The point is that IMS provides rudimentary identification of chemical species giving it advantage over a parallel technology, electron captor (ECD). It gives significantly less identification capability when compared to mass spectrometry (MS). Most drugs and explosives have high proton affinity supporting IMS high detection sensitivities to these classes of compounds.
Electron Capture Detector
Electron capture detector (ECD) is a nonspecific detector. ECD use a radioactive beta source to ionize a gas inside a cell. The current at the collector electrode is measured. As chemical species inter the cell they react with the ionized gas decreasing current flow to the detector. As ECD is very sensitive to electronegative chemical compounds and very insensitive to neutral compounds it is considered a selective nonspecific detector.
Chemiluminescence
The Chemiluninescence detector, although specific for oxides of nitrogen, is considered a general nonspecific detector with high sensitivity.
A detector technology generally used to measure the concentration of oxides of nitrogen (NOx). “Nitrogen oxide (NO) and ozone (O3) react to produce a characteristic luminescence with an intensity linearly proportional to the nitrogen oxide concentration”, Model 42c Instruction Manual, Thermo Environmental Instruments. Nitrogen dioxide is first reduced by passing over a hot catalyst followed by mixing with ozone. Some additional specificity is achieved by adjusting the catalyst and the catalyst temperature. Chemical species with high nitrogen content will react. Some explosives, energetic materials, and drugs react similarly when reacted with ozone either before or after the reduction catalyst.
Olfaction
Technologies offered under the Olfaction name are more a way of handling data than a specific detector. An array of very small simple materials that differ in chemical properties (i.e. solubility or polarity) will interact or react with a specific chemical species (TNT or cocaine or anthrax) differentially exhibiting a unique response pattern. The more elements in the array the more unique the pattern. Thirty-two elements are common. Chemical identification is correlated to the patterned response. The marketing term used often is “pattern recognition”. It can be a very powerful tool.
In some cases an array of polymers that differ in physically properties, primarily solubility is used. The properties are measured electrically, change in resistance. The detector is about the size of a stamp. In other cases, an array of metal porphyrins is used. The pattern that develops upon exposure is read optically. Both of these cases require pattern recognition algorithms for chemical identification.
Frequency Modulated Infrared Spectroscopy (FMIRS)
Infrared spectroscopy is a group of techniques that measure the energy absorbed through molecular bond vibration and molecular rotation. Every chemical species has a unique IR fingerprint. The significance of these techniques is that chemical structural information is measured positively identifying the chemical when vibrational energies are collected over sufficient range. IR and MS are the two techniques under this heading that provide positive specific chemical identification. The sensitivity is proportional to the number of molecules that interact with the energy source. The number of interactions is increased by increasing the distance the energy source travels through the sample.
FMIRS is not a standard acronym in IR spectroscopy. There are a number of techniques in IR spectroscopy that use frequency or amplitude modulation.
Biotechnology
Microbiologists and geneticists have made great strides genetically engineering microbes that can specifically interact with chemical species, explosives, drugs, viruses, etc. The interaction results in some change in physical property of the supported microbe that can be measured. Most elegant of these is bioluminescence, biological light, emitted proportional to the interaction. Sufficient specificity is often attained for chemical characterization. A microbe must be developed for each chemical species of interest.
Surface Acoustic Wave Resonator (SAW)
A SAW detector is a crystal resonator held isothermal. A sample is passed over the detector. As chemical species in the air passing over the detector, they condense resulting in a change in resonator frequency. The amount of shift is a function of the number or concentration of chemical species in the air above the resonator.